The present invention relates to an apparatus and method for reducing a torque fluctuation of an engine, and, more particularly, to an apparatus and method for reducing the torque fluctuation of the engine that can be used for stratified charge combustion in an engine such as a cylinder injection type engine.
In a typical engine, fuel is injected into an intake port from a fuel injection valve to charge a homogeneous mixture of fuel and air to the associated combustion chamber. An intake passage is opened and closed by a throttle valve, which is operated by manipulating an acceleration pedal. The opening of the throttle valve adjusts the intake air amount (and ultimately the amount of homogeneously mixed air and fuel) that is supplied to the combustion chambers of the engine. This controls engine power.
However, when performing homogeneous charge combustion, vacuum is produced by the throttling action of the throttle valve. This increases energy loss due to pumping, which results when the mixture is drawn into the combustion chamber from the intake port. To attempt to solve this problem, stratified charge combustion has been proposed. In stratified charge combustion, the throttle valve is opened wide, and fuel is supplied directly into each combustion chamber. This provides a mixture having a relatively low air-fuel ratio in the vicinity of the ignition plug. As a result, the fuel is easier to ignite.
"Lean burn combustion" includes "stratified charge combustion". "Lean burn combustion" includes not only "stratified charge combustion" but also "homogeneous lean burn combustion," which improves fire propagation with less fuel by forming a swirl or the like in the air-fuel mixture during homogeneous charge combustion. "Stratified charge combustion" and "lean burn combustion" both reduce the amount of fuel supplied to the combustion chambers.
In an engine that performs the aforementioned "lean burn combustion," abnormal combustion due to the self firing of fuel, knocking, may occur. To suppress knocking, the ignition timing is delayed. Delaying the ignition timing, however, reduces the fuel combustion speed, which deteriorates combustion and increases the torque (power) fluctuation of the engine.
For example, Japanese Unexamined Patent Publication No. Hei 4-187851 discloses an apparatus designed to overcome this drawback. This apparatus increases the amount of fuel injection in accordance with the amount of delay when the ignition timing is delayed. The increased amount of fuel injection suppresses the deterioration of the combustion and reduces an increase in torque fluctuation.
When the fuel injection amount is increased, however, the disclosed apparatus increases the likelihood of knocking. To suppress knocking, therefore, the ignition timing should be delayed further. This may result in an over-delay of the ignition timing, which may cause a misfire. Such a misfire would further increase the torque fluctuation.